Quantity the change in which is equal to the heat brought to the system in a reversible process at constant temperature divided by that temperature. Entropy is zero for an ideally ordered crystal at 0 K. In statistical thermodynamics \[S=k\ \ln W\] where k is the Boltzmann constant and W the number of possible arrangements of the system.
Sources:
Green Book, 2nd ed., p. 48 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
PAC, 1990, 62, 2167. 'Glossary of atmospheric chemistry terms (Recommendations 1990)' on page 2187 (https://doi.org/10.1351/pac199062112167)
PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 972 (https://doi.org/10.1351/pac199668040957)